The present invention relates to a power transmission device such as a worm gear serving as a speed reducing mechanism.
First, we shall describe a conventional power transmission device in the form of a worm gear. As shown in FIG. 9, a conventional worm gear comprises a worm 2 having helical threads 1 and a worm wheel 4 having teeth 3 meshing with the threads 1.
As the worm 2 rotates, the threads 1 of the worm 2 engage the teeth 3 of the worm wheel 4, so that the worm wheel 4 will rotate.
With this device, since the threads 1 of the worm 2 mesh directly with the teeth of the worm wheel, the efficiency of torque transmission is low and large energy is required to rotate them.
Also, much heat is produced due to friction. The heat increases the possibility of seizure. Such a worm wheel is thus made of a bronze light alloy (an alloy such as gun metal). Thus, the strength and durability of the teeth 3 were poor.
In order to reduce heat generation and improve durability, it was proposed (in Japanese Examined Patent Publication 38-22759) to interpose balls 5 between the worm 2 and the worm wheel 4 as shown in FIG. 10.
With the conventional worm gear shown in FIG. 10, the gap between a cylinder 7 and the worm wheel 4 had to be kept to a minimum in order to prevent the balls 5 from floating up markedly out of grooves 6a and 6b formed in the worm 2 and the worm wheel 4, respectively. For this purpose, as shown in FIG. 11, the worm 2 had an hourglass shape corresponding to the radius of curvature of the worm wheel 4. But this structure has a drawback in that, since the diameter of the worm 2 is greater at its ends than at its central portion, the inclination of the ball rolling groove 6a in the worm 2 changes from the center toward both ends as shown in FIG. 11. Namely, it was difficult in design to form the groove 6a in the worm 2 and the grooves 6b in the worm wheel 4 so that they will coincide with each other. This in turn hampers smooth movement of balls while the gear is being driven.